/Engine/Plugins/Runtime/NetworkPrediction/Source/NetworkPrediction/Public/Services/NetworkPredictionService_Interpolate.inl
We potentially could special case server-controlled independent simulations since they will at least all line up on the same frame boundaries (rep frequency will mean there are still gaps though). But we don't distinguish this (server remote vs local) on simulated clients. Seems simpler to just handle all SP sims the same.
class IIndependentInterpolateService